Applicants"" invention relates generally to the field of control systems and more particularly to an online monitor used to continuously check various data points in a control circuit and record the data when an event occurs.
Chart recorders and oscilloscopes have been available for many years for use as portable test equipment to examine analog, time varying signals over short intervals. Typically, these devices reside in a laboratory or when they are taken into the field, they are hand carried by an individual responsible for troubleshooting equipment, or to examine analog signals of interest. Rarely is such equipment left attached to a piece of equipment to be monitored in the field for long periods of time, for several reasons. Such equipment is typically expensive and often not suited for an industrial environment in which the electrical and ambient conditions are hostile. Conventional chart recorders and oscilloscopes are generally not designed to capture data for long periods of time. For instance, when a typical oscilloscope is triggered, the waveform(s) captured are stored visually on a cathode ray or other screen until such time as another event for which the oscilloscope has been commissioned to examine occurs, at which time the previous waveforms are erased. In the case of chart recorders, the capture and display of events typically takes the form of a paper representation of the analog waveforms captured, making such a device impractical to be left permanently attached to a remote location to be monitored. Someone occasionally must visit the site, change the paper, and retrieve the plots generated.
It would be beneficial in many applications to have an online monitoring device that is inexpensive, and capable of being permanently left in an application, capable of storing captured waveforms electronically and indefinitely. The waveforms could then be retrieved electronically via an industry standard communication network from a central location. As an example of such an application, consider the monitoring of control signals in the large circuit breakers used by electrical utilities in power distribution systems worldwide. A typical large utility in the United States may have thousands of such circuit breakers, each of which must be-periodically tested to determine their suitability for use when called upon to operate to control the flow of electricity in the power grid. Typically, technicians are employed on a permanent basis to travel to the respective locations of such circuit breakers to test the breakers as part of a normal maintenance schedule. To test a circuit breaker, the technician must manually re-route the current around the circuit breaker to be tested, then manually trip and reset the breaker with an oscilloscope or chart recorder attached to the appropriate test points of the circuit breaker control system. The technician then either interprets the data received immediately, or transports the information manually to another location for further interpretation.
In the normal operation of the power grid, circuit breakers are opened and reset in response to naturally occurring phenomena such as a detected overcurrent condition or overvoltage condition caused by a lightning strike. Many such circuit breakers are electrically connected to a local device such as a commercial programmable logic controller. As such, they can be set and reset remotely via commands sent via a communication network to the programmable logic device. If an inexpensive, permanent online monitor could be provided for each circuit breaker that could communicate it""s information via a communication network to a central location, the process of travelling to the site and the steps taken to test the device could be eliminated. Furthermore, if the device were capable of capturing a plurality of signals representing the interesting features of the circuit breaker control circuits, and were capable of storing such waveforms indefinitely, the resulting device could also capture data from legitimate trips and resets of the breaker under normal operating conditions.
One of the difficulties in supplying such an on line monitoring device is that often a single utility company employs several different industry standard communication networks within its own grid. This phenomenon is common in many other industries as well. A single online monitor product that provides the capability to communicate with external equipment such as a network computer via a wide variety of industry standard networks would be advantageous from the standpoint of common programming, inventory management, and fewer individual components.
Another difficulty in applying prior art analog data gathering devices such as chart recorders and oscilloscopes for permanent installation is that the circuit breaker control circuits operate on several different nominal battery voltages with nominal voltages of 48 VDC, 125 VDC and 250 VDC common. In a conventional chart or data recorder or oscilloscope, the logical conditions that initiate the capture of data, henceforth referred to as events, must be custom programmed for each application, or at least for each voltage level. However, in this and many other applications, the appropriate level to initiate data capture is a function of the instantaneous value of another signal. In the circuit breaker application, the various levels at which one would want to begin capturing data are proportional to the voltage of the battery supplying the circuit breaker. An online monitor that provides the capability to define a trip point that is a function of the instantaneous value of another signal would eliminate the requirement to custom program each unit.
While the analog signal conditioning required to monitor circuit breakers is somewhat unique to that application, the basic functions of an online monitor are broadly applicable in industrial, commercial and other applications. For instance, in the circuit breaker-monitoring application, only voltages, some of which are proportional to currents flowing in the circuit breaker control circuitry are monitored. Other applications for such an online monitor are readily envisioned, such as a network of devices placed around the countryside used to monitor seismic activity. Such a device might monitor several axes of vibration or acceleration, temperature, barometric pressure and other parametric values. An online monitor that provides a modular analog signal conditioning unit would allow the basic online monitor concept to be broadly applicable.
Another feature of a monitoring device capable of being networked is an ability to accurately capture the time at which an event occurs, as defined by the internal programming of the monitoring device. This might be particularly important in applications where the propagation of phenomenon is of interest. For example, in the seismic activity monitoring application described above, the ability to accurately capture the time of an event with a high degree of both accuracy and resolution would be useful in determining the propagation of an energy wave generated by an earthquake.
It would be desirable to have an online monitoring device that substantially achieves these objectives, while retaining the intuitive aspects of the prior art.
Accordingly, the principal object of the present invention is to provide an inexpensive online monitor of analog signals that can be left permanently in an application, capable of capturing a user-definable plurality of analog signals upon satisfaction of a user defined trigger condition.
A further object of the present invention is to provide an online monitor that is capable of communicating with a number of different industry standard fieldbus communication standards by selecting an appropriate modular fieldbus communication module.
Yet a further object of the present invention is to provide a single online monitor that is capable of capturing many different types of analog signals by virtue of selecting an appropriate modular analog processing base suited to the user""s application.
Still a further object of the present invention is to provide a single on-line monitor in which the end user can specify a number of different events, defined as the satisfaction of logic conditions under which the online monitor will begin to capture and store user specified channels of data.
A still further object of the present invention is to provide a single online monitor in which the logic condition that triggers the capturing of data is based on the comparison of the instantaneous value of one analog signal to a user definable function of the instantaneous value of another analog signal, or the comparison of the instantaneous value of an analog signal to a constant threshold value.
An additional object of the present invention is to provide an online monitor in which the time at which an event is declared, as defined by the satisfaction of logical conditions based on the instantaneous value of the analog signals to be monitored, is accurately captured by virtue of a time tag.
In the preferred embodiment of the invention, the invention is comprised of an online monitor control system of essential elements including, but not limited to a digital signal processing unit, a modular, interchangeable, analog signal conditioning base unit that can be tailored to the specific requirements of the process to be monitored and a modular, interchangeable, communication interface unit. The interface unit permits the digital signal processing unit to exchange data with external equipment such as network computers via a number of industry standard communication fieldbus protocols. The online monitor can be permanently attached to a device being monitored and continuously monitors a set of analog signals. It initiates data capture over a window of time when a user programmable set of conditions is satisfied. The online monitor provides the capability to define the trigger condition under which data capture is initiated by comparing a user defined signal to a function of the instantaneous value of another signal or constant. Data captured is stored indefinitely in internally battery backed memory and the instant at which data capture is initiated is determined accurately via a time tagging function, so the data can be retrieved at a later time.